Tech Tips

Display date and time in the past or future and pop through directories easily.

Show Date or Time, Past or Future

When working on Linux/UNIX platforms, I frequently find it useful to
obtain the date and time in the past or future. Whether scheduling jobs,
searching for files from a certain date or determining the day on which a
certain date falls, countless scenarios need a routine to
compute and display the date and time in the past or future. I searched
for a suitable program, but ultimately had to write one myself. This program is
called showdate. It is written in the C language originally on UNIX and
has been ported over to Linux as well. You can download the code from
the LJ FTP site: ftp.linuxjournal.com/pub/lj/listings/issue163/9877.tgz.

After obtaining the source code, assemble the showdate executable using
an ANSI C compiler (either cc or gcc) as shown:

# cc showdate.c -o showdate

Store the executable in /usr/local/bin or a directory of your choice,
and invoke it without any arguments to obtain usage:

-y [+|-]years: Number of years in the past (-) or future (+) offset from the current year.

-m [+|-]months: Number of months in the past (-) or future (+) offset from the current month.

-d [+|-]days: Number of days in the past (-) or future (+) offset from the current day.

-h [+|-]hours: Number of hours in the past (-) or future (+) offset from the current hour.

-M [+|-]minutes: Number of minutes in the past (-) or future (+) offset from the current minute.

-s [+|-]seconds: Number of seconds in the past (-) or future (+) offset from the current second.

-e: Display the time elapsed in seconds since the UNIX epoch (January 1,
1970 UTC).

-f format: Display the date and time according to formatting directives specified in format.

Options e and f are incompatible. Specifying them together on the
command line terminates the program abnormally. The default output
of showdate can be tweaked with the formatting directives and argument to
-f, which are identical to the ones used by the standard date command.
The important thing is that all computations are performed by taking
either a positive (future) or negative (past) offset from the current
date and time (now), which is its datum.

A good way to become familiar with any tool quickly is to understand how it
is used. For example, the command to display the date and time ten years
ago, relative to now, would be (output of showdate has been omitted as
the results depend on the value of now):

# showdate -y -10

To find out the epoch seconds elapsed for the above scenario, use:

# showdate -y -10 -e

A futuristic date of five years, two months and 23 days from now in the YY-MM-DD
format would be output as shown below. The plus sign is optional for future
dates and the two forms of the command line below are equivalent (the
minus sign is mandatory for past dates):

The options can appear in any order, as long as their contextual usage is
unambiguous; therefore, the command line above could be written as:

# showdate -m 2 -f %Y-%m-%d -d 23 -y 5

The +- offsets can be combined in a single command line; however, mixing
them up can lead to unexpected and erroneous results. If now is January
1st 2003 12:00:00 AM UTC, showdate outputs:

# showdate -m -8 -M 32
Wed May 1 00:32:00 2002

The above command displays the date and time in the past—eight months ago
but 32 minutes from now, while the one below displays the date and time
in the future—8 months from now but 32 minutes ago:

# showdate -m 8 -m -32
Sun Aug 31 23:28:00 2003

The capabilities of showdate can be leveraged to specify subminute job
scheduling times. A quick way to schedule a batch job 12 minutes and
35 seconds from now would be:

# showdate -M 12 -s 35 -f %Y%m%d%H%M.%S | xargs at -f job-file -t

The current date and time is tracked as the number of seconds that have
elapsed since the epoch. This number is stored in a signed long, which
means that on a 32-bit system, the timekeeping will break on Tuesday
January 19, 2038 at 03:14:08 UTC, when the value overflows and becomes
negative. An error is returned if the desired date and time exceeds
this limit as shown here:

# showdate -y 1000
showdate: required time exceeds system limit

The presence of whitespace characters in the formatting directive needs
to be escaped or enclosed in quotes (single/double). So, the command to
display the date and time 18 hours, 30 minutes ago in Year-Month-Day
Hour:Minute:Second format would be:

# showdate -h -18 -M -30 -f "%Y-%m-%d %H:%M:%S"

showdate cannot obtain the date and time by specifying a weekly offset
and by taking a positive or negative offset from any datum, not just
the current date and time. Even though showdate has been tried and
tested rigorously, it is not perfect. And, if anyone encounters a bug or
feels that redesigning the algorithm, implementing coding shortcuts
or efficiently using system resources can improve the program, please
contact me by e-mail at ssahore@yahoo.com.

showdate was designed for computing and displaying the date and time
in the past or future depending on the command-line options, specified
as an offset from the current date and time. The next step would be to
augment showdate to specify weeks and the ability to change its datum.

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